1. Field of the Invention
This invention relates to a photocurable and thermosetting resin composition and a printed circuit board made by using the same, and more particularly relates to a photocurable and thermosetting resin composition which excels in tack-free dryness, adhesiveness, and resolving properties, scarcely produces mist during the thermal curing thereof, enjoys high sensitivity, and excels in storage stability, and to a printed circuit board having a solder resist layer and/or a resin insulating layer excelling in resistance to PCT (pressure cooker test), electrical insulating properties, etc., which are produced by using the composition mentioned above and which reduce the generation of gas during the mounting of the parts thereon.
2. Description of the Prior Art
A solder resist composition is used for the purpose of preventing the bridging of solder and protecting circuits during the soldering of a given part to a printed circuit board. The cured product thereof is, therefore, required to possess such properties as adhesiveness, resistance to chemicals, and electrical properties. In recent years, the demand for the production of printed circuit boards of further decreased weight and increased density of conductor circuits has been finding growing recognition in the printed circuit board manufacture industry. To cope with this demand, a photolithographic developing type solder resist composition, particularly the composition which can be developed with an aqueous alkaline solution has been developed and employed (see JP 1-141904, A, for example).
However, in the case of the solder resist composition which has been heretofore used, since the photo-curing of the depths of a coating film at the time of exposure is inadequate in the image formation, the coating film may exfoliate during the development thereof, which forms a big factor of preventing formation of the image with high resolving properties. Therefore, the fully satisfied material is not found out till now, though the demand for development of the solder resist composition excelling in the curing properties in deep portions at the time of exposure increases.
The formation of a solder resist layer using such a photolithographic developing type composition requires a step of applying a resist composition to the entire surface of a substrate by a screen printing method, a curtain coating method, a spray coating method, a roll coating method, etc., a preliminary drying step for volatilizing an organic solvent in order to allow the contact exposure, an exposure step for cooling the coating film and subjecting it to the contact exposure, a developing step to remove unexposed portions by development, and a thermal curing step for obtaining the sufficient coating properties. Among these steps, particularly the exposure step is the very cumbersome process which comprises exchanging a negative film according to the kind of the printed circuit board, carrying out the position alignment thereof, then drawing a vacuum and subjecting it to exposure to light. Therefore, the shortening of the exposure process becomes a significant factor for improving the productivity and lowering the cost. The high sensitivity of the solder resist composition greatly contributes to the shortening of an exposure process. Under such circumstances, as for the solder resist composition used for a general-purpose electric device, the demand for high sensitivity is increasing. In order to realize such demand for increasing the sensitivity, it is generally considered to add a large amount of a polyfunctional (meth)acrylate compound to the composition. However, if the polyfunctional (meth)acrylate compound of a low molecular weight is added to the composition in a large amount, though the sensitivity will be improved, there is a problem that tack-free dryness (tack-free properties) required for the contact exposure decreases remarkably, and the properties of the cured coating film also decrease.
Furthermore, in the formation of a solder resist layer using such a photolithographic developing type solder resist composition, a volatile component (mist) generated from a resist composition adheres to a hot air circulation type drying oven or to an exposure device in a preliminary drying process accompanied with heating, an exposure process, a thermal curing process, and a soldering process, which becomes the cause of the abnormalities in mounting the parts in the following soldering process and a subsequent gold plating process. Therefore, the demand for the development of a solder resist composition which contains a small amount of ingredients (mist) which volatilizes from the resist composition in the preliminary drying process, the exposure process, the thermal curing process, and the soldering process has been finding growing recognition, but the material which may fully satisfy such a demand is not found out till now.
On the other hand, as a method for the production of a multi-layer printed circuit board, a lamination pressing method has been heretofore known to the art. The lamination pressing method, however, necessitates use of production facilities which are very voluminous and highly expensive and incurs difficulty in forming fine patterns because the plating is carried out on the outer layer during the plating step of through-holes and the thickness of copper deposit increases. For the purpose of overcoming such problems, in recent years strenuous efforts are being continued for the development of a multi-layer printed circuit board having conductor layers and resin insulating layers alternately build up (build-up technique).
One method of such a build-up technique is, for example, a method of manufacturing a multi-layer printed circuit board which forms a resin insulating layer by using a photolithographic developing type composition. This method comprises firstly applying a liquid photosensitive resin composition to the entire surface of a circuit board having conductor circuits formed in advance thereon so that the conductor circuits are buried in the resultant coating film by an arbitrary method such as a screen printing method, a curtain coating method, and a spray coating method, drying the coating film, then superposing a negative film having a prescribed exposure pattern on the dried coating film, irradiating the dried coating film with ultraviolet light for exposure, then removing the negative film from the coating film, subjecting the coating film to a development treatment to form a cured resin insulating layer of the prescribed pattern, subsequently treating the resin insulating layer with a coarsening agent thereby imparting coarsened surface thereto, and thereafter forming a conductor layer as by electroless plating or electroplating.
In such a method of manufacturing the multi-layer printed circuit board having conductor layers and resin insulating layers alternately build up, the photosensitive resin composition to be used is required to exhibit sufficient photo-curing of the depths of a coating film at the time of exposure, and the interlaminar resin insulating layer formed therefrom is required to be excellent in such properties as adhesiveness to a conductor layer, electrical insulating properties, heat resistance, and resistance to chemicals. Further, in the heating step performed after formation of a conductor layer as by electroless plating or electroplating, the volatile component generated from the interlaminar resin insulating layer becomes the cause of inferior adhesiveness of the conductor layer thereto. However, the material which may fully solving such problems is not found out till now.
As for the manufacture of a printed circuit board which is used in the devices of small-amount production such as analytical instrument and a printed circuit board to be manufactured as a trial, the demand is directed to a solder resist composition which can cope with a direct imaging technique which directly forms an image on a printed circuit board according to the CAD (computer aided design) data from a computer. As a light source to be used in such a direct imaging technique, a laser source is preponderantly used. The ultraviolet light of 300-450 nm in wavelength is used as a single wavelength or in combination of several wavelengths, the beam diameter is in the range of 5-15 μm, and the output is about several watts. Since the image is formed by scanning with a width of 5-15 μm while repeating ON-OFF of such a laser beam, the period for forming a resist pattern on one printed circuit board greatly depends on the sensitivity of the solder resist composition. Accordingly, the solder resist composition to be used in the laser direct imaging technique is required to possess the sensitivity higher than that of the developing type solder resist composition to be subjected to the general contact exposure.